This section provides background information related to the present disclosure which is not necessarily prior art.
Inflammation is a host response to infection important for pathogen elimination but which also leads to tissue injury that must be repaired. Accordingly, the inflammation response must be tightly regulated as aberrant or excessive inflammation can also result in tissue injury caused by responding leukocytes. Moreover, prolonged pro-inflammatory stimulation or an inability to resolve acute inflammation can contribute to the pathogenesis of a number of diseases that include chronic obstructive pulmonary disease (COPD), asthma, cancer, atherosclerosis, and autoimmunity. With regard to COPD, continual stimulation through repeated cigarette smoke exposure leads to chronic inflammation that is perpetuated even years after cigarette smoke exposure has terminated. Thus, defining the molecular pathways that lead to inflammation, and the identification of possible points of intervention in these pathways are warranted.
The acute inflammatory response to pathogens initiates with tissue injury and/or exposure of pathogen-derived ligands that engage Toll-like receptors expressed on resident tissue macrophages. Macrophages are innate immune cells that reside in diverse tissues and provide sentinel responses against pathogens or noxious substances by the production of pro-inflammatory mediators, such as vasoactive lipids, cytokines, and chemokines. An immediate consequence of this tissue macrophage response is the recruitment of neutrophils that within hours release their granule contents at the site of infection in an attempt to eradicate or neutralize pathogens. However, this response can also lead to local tissue injury. Recruited macrophages, differentiated from newly arriving monocytes, clear remaining pathogens and short-lived apoptotic neutrophils via phagocytosis and begin tissue repair through the production of angiogenic factors and proteolytic tissue enzymes, such as matrix metalloproteases (MMPs). Under normal circumstances, the acute inflammatory response to pathogen exposure is resolved within days.
In tissues, macrophages use integrins to adhere to integrin ligands found in extracellular matrix (ECM) proteins (e.g. collagen and fibronectin) or expressed on the cell surface of other cells (e.g. ICAM and VCAM). The adhesion of myeloid cell to ECM integrin ligands has repeatedly been shown to promote production of many pro-inflammatory mediators such as prostaglandins, inflammatory cytokines, chemokines, and multiple MMPs. Although the production of prostaglandins is known to be dependent on cyclooxygenase activity, the integrin signaling pathways macrophages use to produce other pro-inflammatory mediators are not well understood, although mitogen-activated protein kinases, Src family, and Pyk2 nonreceptor tyrosine kinases are implicated in integrin signaling. Work from our laboratory has found that macrophage adhesion to fibronectin via the α5β1 integrin in vitro leads to MMP-9 production and is normally inhibited by the Arhgef1 intracellular signaling molecule.
Arhgef1 (Lsc/p115RhoGEF) is an intracellular signaling molecule with expression predominantly restricted to hematopoietic cells. Arhgef1 is biochemically and functionally characterized as both a regulator of G-protein signaling (RGS) and Rho guanine nucleotide exchange factor. RGS proteins act as GTPase-activating proteins for GTP-bound Ga subunits of heterotrimeric G-proteins. Arhgef1 specifically accelerates the inherent GTPase activity of Gα12/13 subunits, thereby terminating signaling from GPCRs that associate with Gα12/13-containing heterotrimeric G-proteins. Arhgef1 also contains a tandem Dbl and pleckstrin homology domain that functions as Rho guanine nucleotide exchange factor specific for RhoA. RhoA participates in a number of cell biological processes including regulating cytoskeletal organization, integrin adhesion, and integrin signaling. We have shown that in B-lymphocytes, Arhgef1 is required for resolving integrin adhesion, and analyses of Arhgef1-deficient mouse mutants have further demonstrated a requirement for Arhgef1 in leukocyte migration and adhesion, consistent with the reported role for Arhgef1 in fibroblast adhesion to fibronectin.